Osteoporosis is a leading cause of disability in the elderly, particularly elderly women. It is well known that human parathyroid hormone (hPTH) and certain analogues are useful in the treatment of osteoporosis.
Parathyroid hormone (PTH) is produced by the parathyroid gland and is involved in the control of calcium levels in blood. It is a hypercalcemic hormone, elevating blood calcium levels. PTH is a polypeptide and synthetic polypeptides containing the first thirty-four residues of PTH may be prepared using the method disclosed by Erickson and Merrifield, The Proteins, Neurath et al., Eds., Academic Press, New York, 1976, page 257, preferably as modified by the method of Hodges et al., Peptide Research, 1, 19 (1988).
When serum calcium is reduced to below a "normal" level, the parathyroid gland releases PTH and resorption of bone calcium and increased absorption of calcium from the intestine, as well as renal reabsorption of calcium, occur. The antagonist of PTH is calcitonin, which acts to reduce the level of circulating calcium. Osteoporosis is a progressive disease which results in the reduction of total bone mass. This often results in fractures of load-bearing bones and the physical degenerations characteristic of immobilizing injuries. Osteoporosis is associated with hyperthyroidism, hyperparathyroidism, Cushings syndrome and the use of certain steroidal drugs. Remedies historically have involved increase in dietary calcium, estrogen therapy and increased doses of vitamin D.
Although high levels of PTH can remove calcium from the bone, low doses can actually promote bone growth.
While the use of PTH is effective in the treatment of osteoporosis by diminishing the loss of bone mass, PTH may exhibit other undesired pharmalogical effects, such as hypertension and smooth muscle relaxation (e.g. relaxation of gastrointestinal organs, uterus, tracheal and vas deferens as well as positive chronotropic and inotropic effects on the heart.
Tregear U.S. Pat. No. 4,086,196, describes human PTH analogues and claims that the first 27 to 34 amino acids are the most effective in terms of activation of adenylyl cyclase. Rosenblatt et al, U.S. Pat. No. 4,771,124 discloses the property of hPTH analogues wherein Trp.sup.23 is substituted by amino acids phenylalanine, leucine, norleucine, valine, tyrosine, beta-naphthylalanine and alpha-naphthylalanine as a PTH antagonist. These modified hPTH analogues also have the 2 and 6 amino terminal amino acids removed, resulting in loss of most agonist activities when used to treat osteoporosis.
Pang et al WO93/06845, published Apr. 15, 1993, describes analogues of hPTH which involve substitutions of Arg.sup.25, Lys.sup.26, Lys.sup.27 with numerous amino acids, including alanine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, histidine, isoleucine, leucine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine or valine. These are claimed to be effective in the treatment of osteoporosis with minimal effects on blood pressure and smooth muscle.
The biological activity of hPTH is reflected in the activation of two second messenger systems, G-protein coupled adenylyl cyclase (cAMPase) and cAMPase coupled and uncoupled protein kinase C(PKC) activity. It has been established that the increase in bone growth, i.e. that effect which is useful in the treatment of osteoporosis, is coupled to the ability of the peptide sequence to increase cAMPase activity. The native hPTH containing only the first 34 amino acids has been shown to have all activities. It is typically shown as: ##STR1##
It is the object of the present invention to produce new hPTH analogues having increased cAMPase activity with minimal side effects.